DE19629260C1 - Electro-optical phase modulator with direction-independent impulse response, arrangement of electro-optical phase modulators and use of an electro-optical phase modulator - Google Patents

Abstract

An electro-optical phase modulator, in particular for use in a fibre-optical gyroscope (FOG), is characterised, as long as the time it takes for the light to make one revolution is made to match the working cycle, in that the same pulse response is ensured in both directions of rotation of the light. For that purpose, the modulation electrodes (13, 14...) are arranged in relation to a common counter-electrode (12) in such a way that the propagation in space and time of the potentials on the modulation electrodes and of the electric fields between the electrodes generates a pulse response which is always symmetrically distributed.

Description

The invention relates to an electro-optical phase modulator with an integrated op table waveguide and on both sides at a constant mutual distance from the modulation electrode arranged along the waveguide along the optical axis the.

Phase modulators of this type are primarily used in fiber optics Sagnac interferometers, which are the actual rotation rate measuring instrument for fiber form optical gyroscopes (FOGs = Fiber Optic Gyroscopes), or as a core element used in other interferometric measuring devices, such as Mach-Zehnder interferometers.

The problem and task underlying the invention are depending but explained below with reference to a fiber optic gyroscope (FOG).

With modern fiber optic gyroscopes, an integrated optical one is often used Chip (IO chip) usually uses an integrated on the input side th polarizer, then one Y-branch and two equally spaced along the  optical axes arranged according to the Y-branch in a specific configuration nete electrodes of two phase modulators, which the two in the En determine the light rays radiated into a fiber spool in the opposite direction modulate ter, explained in more detail below. Different versions tion variants of such phase modulators or digital phase shifters in the publications US 5 137 359, US 5 237 629 and US 5 400 142. A FOG with this type of phase modulator shows sensitivity to via interference signals interspersed in the phase modulator.

The coupling of such interference signals into the one containing the phase modulator MIOC path (MIOC = modulating IO chip) can, as explained below, analyze.

Interference signals that couple into the MIOC path can under certain circumstances would cause bias errors. In the following it will be examined how periodic interference signals affect if the gyro-Ab tactaktts opposite the passage tent of light through the fiber is present. Next causes an increased sensitivity to such coupling ne misalignment and other disruptive effects, such as increased Ran dom walk. However, these effects should not be examined here. To Le to give the opportunity to learn how Sagnac-Interfero works meters with random modulation and closed, resetting control loop To familiarize yourself with the European patents EP 0 498 902 and EP 0 551 537 referred.

In order to be able to record the effects of interference injections in the event of a misalignment, it is sufficient to consider the Sagnac interferometer with the control loop open (cf. FIG. 1). Let T be the sampling clock of the system and at the same time the period of an incoming interference voltage, T₀ be the deviating passage time of the light, ϕ (t) be the phase modulation produced by the modulator and ϕ _s (t) be the Sagnac phase. Neglecting DC components and amplification factors in the detector path, the following applies to the output signal y (t) of the interferometer:

y (t) = cos (ϕ (t) -ϕ (t-T₀) + ϕ _s (t)) (1)

Now suppose that by a suitable, acting in the clock T, the signal ϕ (t) superimposed modulation voltage on a modulation in a known manner made the turning points of the interferometer characteristic and that in each case effective sign of the slope of the characteristic curve by a also in cycle T acting demodulator signal is compensated, then the interferometer approximately by a characteristic curve

y (t) = sin (ϕ (t) -ϕ (t-T₀) + ϕ _s (t)) (2)

be described without modulation and demodulation signals. The closeness Strictly speaking, only applies to T = T₀. For T ≠ T₀ occur in narrow overs additional transients that are not in the above equation are taken into account. Since these transients only increase the Ran dom-Walk, and to simplify the calculation, the validity of (2) also assumed for T ≠ T₀, provided the misalignment is not too great. A further simplification results from linearization of the sine function:

y (t) = ϕ (t) -ϕ (t-T₀) + ϕ _s (t) (3)

This signal is filtered by a filter arranged in the data path and then sampled, the nth sample value y _n being calculated by a weighted averaging in the interval [(n-1) T, nT]. Weighting function is the impulse response h (t) of the filter mirrored on the time axis. There are no contributions outside the interval, even if the impulse response does not disappear there, because demodulated signal components outside the interval mentioned are uncorrelated due to the statistical modulation. So that is

The function h [t] is o.B.d.A. so standardized that

apply. The average yaw rate results from

So that is

In the case of sufficiently stationary signals, the averaging over a sequence x _{n is} independent of an index shift, ie it is

In the second integral of (7), the index n can thus be replaced by n + 1. It let ΔT = T₀-T be the clock detuning. Then it will be

If the ΔT is sufficiently small, ϕ ′ (t) ΔT ≈ ϕ (t) -ϕ (t-ΔT). With this approximation it follows finally

Let ϕ (t) = ϕ (t + nT) be a periodic signal with T. Furthermore let ϕ _s (t) = ϕ _S = const. Then

example

As an example, assume that h (t) = 2 / T for t <T / 2 and h (t) = 0 for t <T / 2. For ϕ (t) apply in the range t ∈ [0, T) ϕ (t) = ϕ₀ for t ∈ [0, T / 4) ν t ∈ [3T / 4, T) and ϕ (t) = -ϕ₀ for t ∈ [T / 4, 3 T / 4 ). Outside the range t ∈ [0, T), ϕ (t) is periodically continued according to ϕ (t) = ϕ (t + nT).

The measured phase is therefore proportional to the relative detuning ΔT / T and the amplitude of the scattering ϕ₀. Assuming 100 ppm (ΔT / T = 10 ^-4 ) as the relative detuning and Ampl = 2π · 10 ^-2 as the amplitude of the scattering, then with a gyro with a 2π rotation rate of 2000 ° / s that is caused by the scattering Bias errors:

The invention has for its object an electro-optical phase mod lator for fiber optic interferometers, especially for fiber optic gyros create, in which the previously observed sensitivity to interspersed th interference signals is completely or at least largely eliminated.

The starting point for the invention is the knowledge that the sensitivity of electro-optical phase modulators of the type mentioned theoretically Can be reduced to zero if the time of the work cycle of the interferometer or Gyro with the orbital period of the light from the first phase modulator via the company coil up to the opposite phase modulator is brought. It was recognized that this requires taking measures fen, which ensure that the phase modulator in both pass directions of light has the same impulse response.

The technical teaching of the invention can thus be for an electro-optical Phase modulator with integrated optical waveguide and on both sides in con constant distance from the optical axis along the waveguide ters arranged modulation electrodes characterized in that the elec trodes are arranged so that the spatiotemporal spread of the potentials the electrodes and the electric field between the electrodes symme generate a distributed impulse response.  

This basic idea of the invention is suitable for both analog and digital phase modulators when used in FOGs.

For a digital phase modulator, the preferred embodiment is that several pairs can be controlled in parallel with regard to their longitudinal extension binary graded electrodes with a binary graded electro between them the arranged counter electrode can be provided, with each binary stage there are two sub-electrodes and the symmetry points of all binary stages match agree, such that the complete electrode arrangement ver a symmetrical shared impulse response generated.

In the following, the conditions are derived and individual designs for Phase modulators explained that according to the invention in both Durchrichrich of light deliver the same impulse response.

In the designs of such phase modulators used today, integrated on optical chips, especially in the digital variants (cf. US 5 137 359) usually the symmetry condition derived below is not met. At high-precision fiber optic measuring devices, especially in FOGs, is there with the necessary adjustment of the sampling clock time to the light passage tent possible.

The conditions for an ideal modulated Sagnac interferometer are first described:
With the ideal modulated Sagnac interferometer, the readout function on the photodetector after demodulation is omitted, regardless of the modulation method, as shown above:

y (t) = αu (t) -αu (t-T₀) + ϕ _s (t) (14)

Here u (t) is the reset voltage acting on the phase modulator (or interference voltage), α the electro-optical transmission factor, ϕ _s (t) the Sagnac phase and T₀ the light throughput time from the center to the center of the phase modulators through the coil of the FOG . In the following

u (t) = u (t + T) (15)

a periodic interference voltage with the operating cycle T. With this, and with ϕ _s = 0

y (t) = α (u (t) -u (t-T₀ + T)) (16)

If T = T₀ is ideal, y (t) = 0.

The structure of a real fiber optic interferometer is shown in Fig. 1 of the accompanying drawings.

The light coming from a light source D is at a Y-branch Y in split two parts, which then the modulators m₁ and m₂, then mutually nig the coil S and then again go through the two modulators m₁, m₂. The light rays are under a mutual phase shift

ϕ = ϕ _m + ϕ _s (17)

reunited, where ϕ _{m is} the phase generated by the modulators and ϕ _{s is} the Sagnac phase. Both modulators are controlled by the same voltage u (t) on. The duration of the light from the center of the modulator m₁ to the center of the modulator m₂ be T sei. Then for the phase ϕ _m with opposite polarity of the two modulators:

ϕ _m = α₁⁺ (t) _* u (t) + α₂⁺ (t) _* u (t) -α₁⁻ (t) _* u (t-T₀) -α₂⁻ (t) _* u (t-T₀) (18)

Here, α _n ⁺ (t) is the electro-optical impulse response of the modulator m _n (n = 1, 2) in the direction from right to left, while α _n ⁻ (t) is the electro-optic impulse response of the modulators for the direction of movement from left to right is. The asterisk * indicates the folding:

α (t) * u (t) = ∫ α (τ) u (t-τ) dτ (19)

If the interferometer is now operated with T = T₀ and with a periodic one in T. Voltage u (t) is applied, results

y (t) = (α₁⁺ (t) + α₂⁺ (t) -α₁⁻ (t) -α₂⁻ (t)) _* u (t) (20)

The invention and advantageous details are described below with reference to the Drawing explained in more detail. Show it:

1 shows the above briefly described basic structure of a real Sagnac interferometer.

Fig. 2 is an electro-optical phase modulator, according to the invention as a system with distributed impulse response;

Fig. 3 is a schematic illustration of the electrode assembly for a basic additional variant of the solution of an electro-optical digital Phasenmo Demodulator with optimized distributed impulse response according to the invention;

Fig. 4 shows another embodiment of a basic electrode driving North voltage for a digital phase modulator, the spatial-temporal propagation of applied potentials corresponding to with respect to the Symmetrieanforderun gene according to the invention; and

Fig. 5A, B shows a schematic illustration, the electrode assembly analog Pha senmodulatoren according to the invention, wherein Fig. 5A is a spiegelsymme tric and Fig. 5B illustrative a point-symmetrical electrode arrangement ver.

Phase modulators which correspond to the invention are in principle constructed as shown in FIG. 2. The optically active area, characterized by the rectangle R shown, runs along the x-axis extending from right to left, its extent being limited by the interval [-x₀, x₀] be. The control voltage u (t) now couples with an impulse response h (x, t) that is dependent on x into every point of the active area and generates an incremental phase shift there. If ν is the speed of light propagation in the active area, the electro-optical impulse responses will be:

and

The goal is to make y (t) zero in (20). For this there are according to the invention two possibilities. In the first case you choose

α _n ⁺ = α _n ⁻ (23)

This is fulfilled for (21) and (22)

h _n (x, t) = h _n (-x, t) (24)

The second option leads to

α₁⁺ = α₂⁻ (25)

It follows

h₁ (x, t) = h₂ (-x, t) (26)

which also automatically

h₂ (x, t) = h₁ (-x, t) (27)

and thus

a₂⁺ = α₁⁻ (28)

is fulfilled. From this it follows that y (t) = 0.

The symmetry requirements for an electrode layout that meets the conditions of the invention are explained below:
The phase modulator of the type according to the invention, in particular for a fiber optic gyroscope, can be produced as an integrated optical component (chip), an optical waveguide being diffused into a suitable material, in particular LiNbO₃ or LiTaO₃. This waveguide has an optical refractive index that is dependent on an applied electric field. The necessary electrical field is generated by the electrodes arranged parallel to the waveguide on the surface of the module.

An electrode arrangement corresponding to the first case explained above (equation (23)) is shown in FIG. 3 for an active channel 1 of the symmetrically constructed pair of phase modulators m 1 or m 2. 2 denotes electrode connections for the binary-controllable electrodes of the digital modulator. Be train note 4 denotes a two modulators m₁, m₂ assigned to the counter electrode.

The distributed impulse response h (x, t) is therefore dependent on the spatiotemporal distribution of the generated electric field. The symmetry requirements derived in the preceding section for the distributed impulse responses h _n (x, t) can be met by symmetrical electrode arrangements on the phase modulator, the spatial-temporal propagation of the potentials on the electrodes also having to satisfy the symmetry requirements. This applies both to digital ( FIGS. 3 and 4) and to analog modulators ( FIGS. 5A, B).

In the case of digital modulators (cf. FIG. 3), a separate electrode arrangement is provided for each bit value, the weightings being realized by corresponding area ratios. The symmetry conditions must be fulfilled for each individual bit, the points of symmetry of all bits having to match, so that the symmetry condition for the complete electrode arrangement is ultimately met.

For the two solutions mentioned, digital modulators result therefore the following arrangements:

In the first case, the modulators must have an electrode layout that is mirror-symmetrical to the axis x = 0, so that a spatial-temporal symmetrical spread is also guaranteed to this axis. For a digital modulator, this results, for example, in the layout shown in FIG. 3. The optical waveguide is represented by the arrow running in the middle between the electrodes (active channel 1 ). Only one of the two modulators is shown, the other must be designed accordingly.

The second case leads to an electrode layout in which the electrode geometries of the two modulators emerge from one another by rotation by 180 °. Fig. 4 shows the basic diagram of such an electrode layouts. In the prin zip representation is with 10 the active channel of the modulator m₁, with 11 the active channel of the modulator m₂, with 12 the common counter electrode, with 13 the bi nary electrode array of the modulator m₁ and with 14 that in its arrangement by 180 ° rotated binary electrode array of the modulator m₂.

The corresponding ratios for the mirror-symmetrical or the point-symmetrical design of the electrode layout for analog phase modulators of the type according to the invention can be directly recognized by the person skilled in the art from FIGS .

The invention allows at least two types of electrode layers outs for Sagnac interferometers, both from analog and digi tal phase modulators, so realize that with ideal clocking the influence of periodic interference signals is reliably suppressed.

Claims (7)

1. Electro-optical phase modulator with integrated optical waveguide and arranged on both sides at a constant mutual distance from the optical axis along the waveguide modulation electrodes, characterized in that the electrodes ( 2 ; 13 , 14 ) are arranged so that the spatial temporal spread of the potentials the electrodes and the electrical field between the electrodes, a symmetrically distributed impulse response, it produces. 2. Electro-optical phase modulator according to claim 1, characterized can be controlled by several pairs in parallel with regard to their area ratio Binary graded electrodes and a binary graded between them Electrodes arranged counterelectrode, each binary stage consisting of two parts troden exists and the symmetry points of all binary levels match, the art that the complete electrode arrangement a symmetrically distributed Im pulse response generated. 3. Arrangement with at least one electro-optical phase modulator Claim 1 or 2, characterized in that two phase modulators are integrated together with a Y-branch in an IO chip. 4. Arrangement according to claim 3, characterized records that the arrangement of the modulation electrodes of the one phase mo dulators (m₁) opposite the arrangement of the modulation electrodes of the other Phase modulator (m₂) is rotated by 180 °. 5. Arrangement according to claim 3 or 4, characterized ge indicates that a polarizer is also integrated in the IO chip. 6. Arrangement according to claim 5, characterized records that the polarizer by proton exchange technology together with the integrated waveguide in a LiNbO₃- or LiTaO₃ substrate generated has been. 7. Use of an electro-optical phase modulator Claim 1 or 2 or an arrangement according to one of claims 3 to 6, characterized in that this or these part (s) of the opti structure of a fiber optic Sagnac interferometer.